1. Field of the Invention
The present invention relates to an apparatus for determining sleep staging (the depth of sleep), particularly during anesthesia in a therapeutic/surgical operation.
2. Description of the Related Art
Sleep staging, both in physiological and in medicinally induced sleep, is usually determined visually by experienced experts who evaluate a patient's EEG and allocate values, which reproduce sleep staging, to individual periods of time lasting 20 to 30 seconds. Evaluation is based on work by Rechtschaffen & Kales from 1969 and is nowadays the only generally recognized standard for assessing and ascertaining human sleep staging on the basis of an EEG.
It is evident that this technique is unsuited to being performed on a fairly large scale or during a therapeutic/surgical operation for which the patient is put into a medically induced sleep (anesthesia). For this reason, numerous tests have been performed to determine sleep staging on the basis of the EEG signal. In particular, EEG signals were transformed (fast Fourier transformation) for this purpose and the representation of the EEG signal in the frequency range underwent evaluation. Particular attention was placed on frequencies which were generally designated as A waves and ranged from about 0 to 3.5 Hz. These EEG signal portions of a very long wavelength were associated with the state of consciousness known as "sleep" and methods were developed which were intended to enable determination of sleep staging on the basis of these frequencies. The results were verified by comparing the automatically determined sleep staging with the sleep staging visually determined by experts according to the recognized Rechtschaffen & Kales technique.
The methods so far used did not, however, produce any reliable results or produced results that could only be applied with considerable qualification, so that ultimately a reliable, fully automatic determination of sleep staging is not possible to this very day.
In addition to the aforementioned efforts to determine the state of consciousness known as "sleep" in a universally valid manner that can be automated, there were experiments to evaluate medically induced sleep in particular. To an extent individual frequency ranges were also examined beyond the A wave range. Moreover, success here was only moderate and was restricted to the particular drug being examined, so that it was impossible to apply the results to other drugs. The validation of the results by means of a visual assessment based on Rechtschaffen & Kales also proved deficient, thus making it hardly possible to speak of a reliable evaluation of human sleep.
Knowledge about sleep staging is not just of interest to scientific or medical studies and applications. The absence of medically induced sleep during a therapeutic/surgical operation represents discomfort for the patient which is scarcely conceivable for anyone not affected. This applies all the more so if in addition to the absence of medically induced sleep, the medically induced insensitivity to pain is not achieved either and if at the same time the paralyzation of muscular activities, which is regularly provided during anesthesia, is successfully induced. As far as patients are concerned, this ultimately means that a therapeutic/surgical operation is performed without their being asleep and insensitive to pain, but without their ability to draw attention to their condition. This situation can be remedied by a reliable identification of sleep which is performed fully automatically and which also runs its course during a therapeutic/surgical operation and the anesthesia entailed thereby.